That's what I suspected. This was when the rainbow became a sign to remind
God of His covenant. IIRC, rain in ANE Cosmology came from vaults through
windows in the firmament.
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Stephen Krogh said:
"Did the text state that this rainbow was the first rainbow?"

Answer, Gen. 9 (NIV)
8 Then God said to Noah and to his sons with him: 9 "I now establish my
covenant with you and with your descendants after you 10 and with every
living creature that was with youâ€”the birds, the livestock and all the wild
animals, all those that came out of the ark with youâ€”every living creature
on earth. 11 I establish my covenant with you: Never again will all life be
cut off by the waters of a flood; never again will there be a flood to
destroy the earth." 12 And God said, "This is the sign of the covenant I am
making between me and you and every living creature with you, a covenant for
all generations to come: 13 I have set my rainbow in the clouds, and it will
be the sign of the covenant between me and the earth. 14 Whenever I bring
clouds over the earth and the rainbow appears in the clouds, 15 I will
remember my covenant between me and you and all living creatures of every
kind. Never again will the waters become a flood to destroy all life. 16
Whenever the rainbow appears in the clouds, I will see it and remember the
everlasting covenant between God and all living creatures of every kind on
the earth." 17 So God said to Noah, "This is the sign of the covenant I
have established between me and all life on the earth."

Evidently, in ANE (Ancient Near East) understanding, they thought clouds
brought the rainbow rather than sunlight/rain.

â€śWhenever I bring clouds over the earth and the rainbow appears in the
clouds, 15 I will remember my covenant between me and you and all living
creatures of every kind.â€ť